1. Field of the Invention
The present invention is directed to a method and system for telecommunications and, in particular, for dispersions maps in long haul and ultra-long haul wavelength division multiplexed optical fiber systems with enhanced distributed gain.
2. Related Art
Over the past decade, long-haul data transmission capacity has greatly expanded. This capacity expansion is due to a series of technological developments including erbium doped fiber amplifiers (EDFA), high speed time division multiplexing (TDM), wavelength division multiplexing (WDM), and non-zero dispersion-shifted optical fiber.
However, transmission capacity is ultimately limited by the interplay of transmission impairments (i.e., the degradation of fidelity of the optical data carrier signal) caused by several fundamental physical phenomena, including attenuation, Rayleigh scattering, dispersion, and optical nonlinearity of the fiber. Each of these known impairments will now be discussed.
Attenuation:
Though the glass used for optical fiber is highly transparent at the wavelengths of radiation used for optical data transmission (about 1250 nm to 1650 nm), fundamental physical processes, such as Rayleigh scattering and Erbach-tail absorption, can cause an exponential decay as a function of fiber length in the energy per bit of an optical data signal. This attenuation is generally greater than 0.15 dB/km for even the most nearly ideal silica-based optical fiber. In the absence of any mechanism for reamplifying the optical signal, this attenuation would limit data transmission to a maximum distance of about 500 km or less. The development of optical amplifiers, such as the EDFA, have enabled transmission over much longer distances by periodically boosting the optical signal to overcome the attenuation.
However, amplifiers can introduce noise in the form of amplified spontaneous emission (ASE) which degrades the optical signal. The degree to which the amplification degrades the signal is determined by the physical properties of the amplifier and also by the total attenuation of the signal prior to amplification, i.e., the distance between amplifiers.
An alternative to EDFAs, is to use distributed amplification. For example, distributed Raman amplification (DRA) involves launching an optical pump signal, along with the data signal (conventionally counter-propagating) into the fiber composing the transmission span. The wavelength and power (or intensity) of the pump signal is selected to induce stimulated Raman scattering (SRS) within the fiber, so as to amplify the data signal. Contrary to the case for an EDFA, which is essentially a discrete device, the amplification based on SRS may be arranged to be distributed over a large fraction of the transmission span between repeaters.
For example, FIG. 1 shows a plot of relative signal power (in dB) as a function of distance (km) for EDFA versus DRA. FIG. 1 compares the evolution of the signal power over a typical repeater span distance (about 75 km) for transmission with EDFA based (discrete) repeaters and DRA. The degradation of the data signal by ASE noise will be greater in the EDFA case as the maximum loss [minimum signal power] is greater [less] than for the DRA case.
Dispersion:
The speed of light (as measured in group velocity) in a material such as silica optical fiber varies significantly with the particular wavelength of the optical signal. This phenomenon is known as group velocity dispersion (GVD, or also referred to as group delay dispersion, GDD). This GVD affects transmission of an optical data signal as the signal must be comprised of a band of wavelengths in order to carry information.
For example, a pulse of light representing an isolated xe2x80x9c1xe2x80x9d bit will be composed of wavelengths with a spectral bandwidth approximately equal to the inverse of the temporal duration of the pulse. After propagation over a full transmission link, if the total group delay for the shortest wavelengths differs from the delay for the longest wavelengths by more than about one bit period, then a significant fraction (e.g.,  greater than 25%) of the energy for that xe2x80x9c1 xe2x80x9d bit will spill over into the time slots of neighboring xe2x80x9c0xe2x80x9d bits. This xe2x80x9cspill-overxe2x80x9d results in inter-symbol interference (ISI), whereby the values of the xe2x80x9c1 xe2x80x9d bit and its neighbors may be determined erroneously at the terminus of the transmission link (such as at the receiver of a conventional transmission span).
For ideal linear transmission (i.e., neglecting the nonlinear impairments described below), the ISI may be eliminated by arranging for the total dispersion of the transmission link to be essentially zero. To this end, optical fibers have been developed with very low dispersion in the wavelength range of interest, such as dispersion shifted fibers (DSF). However, this particular approach has proven disadvantageous due to nonlinear optical effects.
Alternatively, dispersion compensating fibers (DCF) have been developed to cancel the dispersion of standard single-mode fibers (SMF, i.e., fibers with a zero-dispersion wavelength of about 1310 nm). Transmission spans have also been developed consisting of so-called non-zero dispersion shifted fibers (NZD). For this approach, two types of fibers are alternated in the link. The two fibers are similar in design to the DSF fibers, but with small, non-zero dispersions in the bands of the alternating wavelengths approximately equal in magnitude but of opposite sign; the magnitude of the dispersions are midway between the SMF and DSF fibers.
Nonlinearity:
Another important class of transmission impairments results from optical Kerr nonlinearity. This nonlinear effect occurs because the index of refraction of the silica fiber transmission medium depends on the intensity of the light being transmitted through the fiber. For a multi-channel transmission system, where the optical power is distributed over a very large number of wavelengths, the Kerr effect can cause the following nonlinear optical phenomena:
(1) Self-phase modulation SPMxe2x80x94This nonlinear phenomenon is a broadening of the bandwidth of an optical channel due to its own power. This phenomenon impairs transmission by exacerbating dispersion induced ISI, and can cause interchannel crosstalk in WDM systems with close channel spacing.
(2) Cross-phase modulation (XPM)xe2x80x94This phenomenon is a broadening and/or shifting in frequency of an optical channel induced by the intensity of the other channels. Impairments due to XPM are qualitatively similar in effect to SPM, though they may be quantitatively dominant for systems with large channel counts and close channel spacing.
(3) Four-wave mixing (4WM)xe2x80x94This phenomenon describes the interaction of channels at two separate wavelengths, generating power at a third wavelength, which may overlap and interfere with a third data channel. Four-wave mixing is especially problematic in WDM systems with many channels evenly-spaced in frequency. Particular data channels may be overlapped by mixing products from many pairs of other channels. The effect is equivalent to increasing the noise and/or crosstalk in that channel.
As the Kerr nonlinearity operates on the light intensity (optical power per unit area), these nonlinear effects may be mitigated either by using low optical powers or by using fibers with relatively large effective mode field areas (Aeff), i.e., with a core size that is as large as practical but still single mode. The 4WM is a coherent effect, and so may be mitigated by constructing the transmission link from alternating types of NZD as described previously.
Thus, what is needed is a method and system for optical communications which takes into account each of the aforementioned impairments.
According to a first embodiment of the present invention, a transmission span for a telecommunications link comprises a first segment that includes a first optical fiber having a first fiber length and a first physical property, a second segment that includes a second optical fiber having a second fiber length, and a third segment that includes a third optical fiber having a third fiber length and a third physical property. The first and third segments are optically coupled to opposing ends of the second segment. At least one of the second and third physical properties can be different from the first physical property. The first segment can provide low non linearity, the third segment can provide distributed gain, and the second segment can compensate for the dispersion of the first and third segments. Alternatively, the first and third segments can provide low nonlinearity and the second segment can provide a primary Raman gain medium for the span.
Further, a dispersion condition (Dc) for the span can be expressed by:             D      c        =                  1        L            |                                    D            1                    ⁢                      L            1                          +                              D            2                    ⁢                      L            2                          +                              D            3                    ⁢                      L            3                              |              ≈        Δ              ,
where D1 is a first dispersion coefficient for the first segment, L1 is the first fiber length, D2 is a second dispersion coefficient for the second segment, L2 is the second fiber length; and D1 is a third dispersion coefficient for the second segment, and L3 is the third fiber length, and L is the total span length. A dispersion slope condition (Dxe2x80x2c) for the span can be expressed by:             D      c      xe2x80x2        =                  1        L            |                                    D            1            xe2x80x2                    ⁢                      L            1                          +                              D            2            xe2x80x2                    ⁢                      L            2                          +                              D            3            xe2x80x2                    ⁢                      L            3                              |                        ·                      δλ            2                          ⪡        Δ              ,
where Dxe2x80x21 is a first dispersion slope for the first segment, Dxe2x80x22 is a second dispersion slope for the second segment, Dxe2x80x23 is a third dispersion slope for the third segment, xcex4xcex is a total wavelength bandwidth communicated by the span, and 0xe2x89xa6xcex94xe2x89xa61.0 ps/nm/km. The span can further include additional segments as well. Each of the segments can be the same length or different lengths from each other.
Alternatively, the span can include two different fiber segments, where the first segment provides low nonlinearity and the second segment provides primary distributed gain and compensates for the dispersion of the first segment.
According to yet another embodiment of the present invention, a telecommunications system for communicating an optical signal, comprises a first transmission span the same or similar to that described above, a first line unit disposed at a first end of the first transmission span, and a second line unit disposed at a second end of the first transmission span. In one aspect, a Raman pump can be provided at the second and/or first line units to introduce a Raman pump signal into the span, preferably for distributed Raman amplification. The first transmission span can be part of a transmission link that includes multiple repeating spans having the same arrangement of the first transmission span.
According to yet another embodiment of the present invention, a method of providing a transmission span that compensates for signal attenuation, dispersion, and nonlinearity of an optical signal communicated between two line units, includes dividing the transmission span into a plurality of fiber segments, and selecting a fiber for each of the segments so that a first segment provides low non linearity, a third segment provides distributed gain, and a second segment compensates for the dispersion of the first and third segments, with the dispersion based on a dispersion condition and a dispersion slope condition for the span. In one aspect, these conditions can be determined based on of the effective mode field areas of the fibers selected.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings.